Question: In history class, the girl to boy ratio is $4$ to $3$. If there are $20$ girls, how many boys are there?
A ratio of $4$ girls to $3$ boys means that there is a group of $4$ girls for every group of $3$ boys. If there are $20$ girls, then there are $5$ groups of $4$ girls each. Because we know the ratio is $4:3$ , there must be $5$ groups of $3$ boys each. There is a total of $15$ boys in history class.